Teaching operating system internal courses has own downside, you get to learn about cool mechanisms that are not often used in practice. One of them is custom pagers. The idea is to supply to the operating system a special function to handle virtual memory page-ins and page-outs. These functions could be used to load and store the pages into a different backing store,compress the data, of even generate it as needed.
One use of custom pagers is for encrypting data, and it seems that Mac OS X uses this to protect some executable (notably the finder). This interesting article explains how a custom pager is used to handle protected Mach-O binaries. Actually, this mechanism seems to have been hacked to a certain degree, which is not that surprising, the OSX86 blog claims to provide the patches for running the kernel of 10.4.8 in command line mode, some other files admittedly enable full GUI loading.